The production of silicon integrated systems, be they for power or for processing, is done increasingly with differential structures of variable reference impedance for the analogue portions of the system. The “outside” world remains, however, essentially a system of the common-mode type and of reference impedance 50 Ω.
The link between a symmetric transmission line and an asymmetric transmission line may not be produced without a suitable electrical circuit. This transition is handled by a transformer of the symmetric-asymmetric type also called a “balun”.
A balun converts for example a signal of common-mode type into a signal of differential-mode type, and vice versa, and handles the impedance transformations. The main electrical characteristics of a balun are its insertion loss, which must be as low as possible, the balancing of the differential channels in phase (180°) and in amplitude (δ=0 dB), and its passband, that is to say the frequency span over which the transformer is usable as a “balun” with balancing of the channels in phase and in amplitude.
Baluns can also be used for example in reception and transmission circuits of wireless communication systems, for the design of differential circuits such as amplifiers, mixers, oscillators and antenna systems.
Though the insertion loss can reduce certain aspects of performance, such as the electrical efficiency or the gain, for example, the unbalancing of the differential channels can lead to a complete malfunction of the circuit which results in a hike in the levels of the even harmonics, significant unbalancing of the differential channels, and increased impact of the connection elements on the outside world (e.g., bonding ground).
Baluns can be produced with transmission lines such as Lange couplers, couplers of circle-shaped type commonly designated by the person skilled in the art under the name “Rat-race”, Marchand couplers, else with stacked or coplanar inductors.
In transmission line structures, lines of length equivalent to a quarter wavelength, λ/4, or a half wavelength, λ/2, are used for the insulations between channels and to delay one channel relative to another. By way of example, for frequencies of 2 GHz and 80 GHz, and for a metal line having a dielectric constant εr equal to 4, the length values λ/4 and λ/2 correspond to 18.75 mm and 37.5 mm for 2 GHz, and 0.468 mm and 0.937 mm for 80 GHz.
Consequently, this transmission lines structure exhibits the drawback of occupying a large silicon area for microwave applications or of not being integratable for lower-frequency applications. For this reason, inductor-based structures are preferably adopted.